All the sites told me I needed 3.5"s for my 83 Bonneville, and it really took a single 4x10 in the center front. The 3.5's in my friend's MC are in the front left and front right corners, and seem fairly easy to get out. I have the Infinity 3.5's in the front, and the pioneer 4x10's in the rear, they sound ok, you're not going to get very strong bass or midbass, but amplification helped them a little. If you're wanting to get real bass, good luck with anything from the trunk too, even with the 15L7, I couldn't really hear anything until it got moved into the actual cabin.
Bass blockers work good, but if you have a decent amplifier you can use the high pass filter and limit the lower frequencies to the front speakers, works just the same, and allows the speakers to get louder without damaging them.